Metastatic breast cancer is progressive and incurable, but the analysis in Cancer Epidemiology, Biomarkers and Prevention found that more than 17 percent of women under age 64 whose metastatic breast cancer was diagnosed between 2000 and 2004 survived for 10 years or longer.

From 1992 to 1994, the five-year survival rate for women under age 49 with newly diagnosed metastatic breast cancer was 18 percent. From 2005 to 2012, five-year survival had doubled to 36 percent.

Clearly, improvements in treatment are part of the explanation. But better imaging techniques have also resulted in earlier detection of metastatic disease and may help explain the increase in years of survival.

In fact, although women are surviving longer, the incidence of metastatic breast cancer has been increasing, especially in young women.

“It’s difficult to know the contribution of early imaging on one hand and better treatment on the other,” said the lead author, Angela B. Mariotto, a statistician with the National Cancer Institute.

“More recent use of imaging may contribute to some of these improvements, because women are diagnosed earlier.”

But treatment has improved, too. Dr. Mariotto noted in particular the introduction of trastuzumab (Herceptin) in 1998 for the treatment of certain tumors, which has significantly improved long-term survival.

“We know a lot about the incidence of metastatic breast cancer and a lot about death from the disease,” Dr. Mariotto said. “But we don’t know much about what happens in between. This data fills some of the gaps.”

Dr. Mariotto is optimistic about breast cancer treatment and survival. “Even though it’s difficult to treat and impossible to cure,” she said, “still it’s becoming more a chronic condition.”